Dual shearing mechanism and method

ABSTRACT

Cutting apparatus including tandemly mounted first and second pairs of juxtaposed knives, each pair of said knives being disposed for passage therebetween of material to be cut, means for urging said first pair of knives against said material when disposed therebetween to score said material, means for mounting said second pair of knives in alignment with the scoring produced by said first pair of knives, and means for urging said second pair of knives toward one another for parting said material at said scoring when said material is disposed between said knives.

United States Patent 72] Inventor John J. Sutyak Pittsburgh, Pa. [211App]. No. 747,156 [22] Filed July 24,1968 [45 I Patented Mar. 2, 1971[73] Assignee Mesta Machine Co.

Pittsburgh, Pa.

[54] DUAL SHEARING MECHANISM AND METHOD 13 Claims, 8 Drawing Figs.

[52] [1.8. CI 83/7, 83/8, 83/9, 83/407, 83/503 [51] Int. Cl. B26d H24[50] Field ofSearch 83/7, 8, 9, 51, 407, 503,169,10-12, 923, 45, 39

[56] References Cited UNITED STATES PATENTS 2,109,921 3/1938 Leach83/169X 2,609,049 9/1952 Rayburn 83/9X 3,080,783 3/1963 Knepshield...83/503X 3,364,803 l/l968 Senftleben 83/503 3,453,914 7/1969 Lemper et al83/8 3,459,086 8/1969 Reeder, Jr. 83/503X Primary Examiner-James M.Meister AttorneyBuell, Blenko & Ziesenheim ABSTRACT: Cutting apparatusincluding tandemly mounted first and second pairs of juxtaposed knives,each pair of said,

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DUAL SHEARING MECHANISM AND METHOD The present invention relates to adual rotary shear and more particularly to shear mechanism, of thecharacter described adapted for cutting thick steel having a thicknesswhich precludes use of conventional rotary shears. The inven tioncontemplates also scrap or trimming disposal apparatus used with theshear mechanism of the invention.

Although my rotary shear is of general utility in severing or cuttingrelatively thick steel plate, the invention will be described inconnection with thick steel plate, the invention will be described inconnection with lateral edge trimming in a conventional rolling mill. inthis application, my novel rotary shear can be arranged to trim one orboth lateral edges of moving plate while the rolling mill is inoperation, if desired. Exemplary arrangements of my rotary shearmechanism can be provided with apparatus for continuousdisposal of plateside trimmings as the plate is passed through the rolling mill. Mostimportantly the operating power requirements of my novel shear mechanismare drastically reduced.

A number of rotary trimmers have been disclosed previously for usetrimming the lateral edges of steel strip or the like as it passesthrough a rolling mill. Known rotary shears, however, have been usefulonly with strip material and relatively thin plate. Because of theirpower requirements, conventional rotary shears can accommodate a maximumthickness of about three-fourths inch of plate. As this thickness isapproached, conventional rotary shears impose severe speed limitationsupon rolling mills. For thicker plate, a guillotine-type shear must beused which further limits the speed of the rolling mill.

With either type of known shearing mechanism, the trimmed lateral edgesof the strip cannot be maintained within acceptable tolerances,particularly at higher operating speeds of the mill. For certainapplications, it has been necessary to retrim the lateral strip edges orto employ separate and subsequent cutting operations.

The shearing mechanism of my invention is more compact than aconventional guillotine shear line, presently required for shearingplate of the thickness which can be handled by my novel rotary shearingmechanism. My shearing apparatus is capable of cutting or trimmingthicker plates with greater speed and less power consumption thanhitherto possible with any known form of shear mechanism.Notwithstanding its greater speed capability, the shearing apparatus ofthe invention can hold closer cutting tolerances. Certain forms of myshearing mechanism are provided with novel apparatus for disposing ofthe resulting scrap. Still other arrangements of my shearing mechanismsimultaneously and accurately trim both lateral edges of a relativelythick steel plate as it passes through the rolling mill.

1 am, of course, aware of a number of previously proposed rotary shearscapable of handling strip and relatively thin plate. For example theU.S. Pat. to Knepshield Nos. 3,080,783; Clayton 2,309,045; Nash2,203,738; Grevich 3,191,474; and Jones 3,257,887 disclose the use ofcutting heads involving a single opposed pair of rotary shears orknives. Certain of the patented apparatus are arranged forsimultaneously trimming both lateral edges of thin strip or platematerial as it passes through the rolling mill, for example U.S. Pat.No. 3,080,783. The latter patent also discloses (FIG. 4) the use of asingle pair of appropriately mounted spare rotary shears for quickreplacement. None of these patents however are capable of trimming thelateral edges of or otherwise cutting relatively thick plate. Of equalsignificance is the lack of close tolerance control demanded bysubsequent, presentday manufacturing techniques.

The U.S. Fat. to Wickwire -Nos. 2,702,597; Repper 2,814,345; Karber3,060,778; and Louehrs 2,318,173 show various types of rotary cuttersemployed for the most part in chopping steel scrap. U.S. Pat. No.2,318,173 utilizes rotary choppers for severing thin sheets or stripinto selected lengths.

None of these patents however are adaptable for chopping relativelythick plate trimmings or the like for scrap disposal purposes.

The U.S. Pat. to Bottenhorn Nos. 2,679,900; Thomas 1,619,684; Cohen2,307,452; Hall 2,706,524 for the most part show the use of multipleannular knives and spacers on a single arbor for multiple slitting orfor combined slitting and side trimming operations. These patents arelikewise only adaptable to cutting, slitting or trimming relatively thinplate or strip material.

I overcome these disadvantages of the prior art by providing a novelrotary shearing mechanism and method involving the use of first cuttingmeans for relatively deeply scoring the plate of my stepwise process andapparatus permits cutting of much thicker plate material than wastherefore possible. At the same time the cutting operation is faster andmore accurate than that possible with any type of conventional shear. Byfirst deeply scoring the plate material, the work hardened surface orskin layers of the material are penetrated to facilitate the followingshearing or parting operation. My apparatus and method, therefore,permit much faster and more accurate cutting operations than previouslypossible. In addition my apparatus and method can be used in conjunctionwith high speed plate rolling mills and are capable of suitable speedmatching as expanding technology leads to further increases in rollingmill speeds. My method and apparatus can be adapted with equal facilityto trimming one or both lateral edges of plate material as it passesthrough a rolling mill.

1 accomplish these desirable result by providing cutting apparatuscomprising first cutting means for scoring material to be cut, secondcutting means for shearing said material, means for mounting said secondcutting means tandemly of said first cutting means in alignment with thescoring of said material produced by said first cutting means, and meansfor passing said material successively to said first and said secondcutting means.

I also desirably provide similar apparatus wherein said material issteel plate having a work hardened skin layer, and said first cuttingmeans are disposed for scoring said plate so as to penetrate said skinlayer.

1 also desirably provide similar apparatus wherein each of said firstand said second cutting means include a pair of juxtaposed rotatablymounted circular knives.

I also desirably provide similar apparatus wherein roll leveler andchopper means are mounted generally in tandem with said first and saidsecond cutting means for disposing of cuttings or trimmings therefrom.

I also desirably provide similar apparatus wherein said cuttingapparatus is arranged for trimming the lateral edges of strip or platematerial and includes means for mounting said first and said secondcutting means adjacent one of said lateral edges, and additional firstand second cutting means are mounted adjacent the other of saidlateraledges.

1 also desirably provide a method for cutting comprising the steps ofscoring material to be cut on intended cut line, and subsequentlyshearing said material along said out line.

I also desirably provide a similar method wherein said material has awork-hardened skin layer, said method comprising the additional step ofscoring said material at least to the depth of said skin layer prior tosaid shearing step.

During the foregoing discussion, various objects, features andadvantages of the invention have been set forth. These and otherobjects, features and advantages of the invention together withstructural details thereof will be elaborated upon during theforthcoming description of certain presently preferred embodiments ofthe invention and presently preferred methods of practicing the same.

In the accompanying drawings 1 have shown certain presently preferredembodiments of the invention and have illustrated certain presentlypreferred methods of practicing the same, wherein:

F IG. 1 is a top plan view of one form of shearing apparatus of myinvention, arranged here for trimming both lateral edges of steel plateor the like. This FIG. shows also scrap disposing and cutting apparatuslikewise arranged in accordance with my invention;

FIG. 2 is a front elevational view of the apparatus as shown in FIG. 1and taken along reference line II-II thereof;

FIG. 3 is a vertically sectioned view of the apparatus as shown in FIG.2;

FIG. 4 is an enlarged partially sectioned partial view of the apparatusas shown in FIG. 3;

FIG. 5 is a partial cross-sectional view of the apparatus as shown inFIG. 3 and taken along reference line V-V thereof;

FIG. 6 is a partial cross-sectional view of the apparatus as shown inFIG. 3 and taken along reference line VI-VI thereof;

FIG. 7 is an enlarged, partial cross-sectional view of the apparatus asshown in FIG. 3 and taken along reference line VII-VII thereof; and

FIG. 8 is another enlarged, partial cross-sectional view of theapparatus as shown in FIG. 3 and taken along reference line VIII-VIIIthereof.

My invention will now be described in detail in connection with anexemplary modification thereof, employed in conjunction with a platerolling mill, for simultaneously trimming both lateral edges of theplate material passing therethrough and for the concomitant disposing ofthe resulting scrap. It will be understood as this description proceeds,however, that my in vention can be employed in other applications foraccurately and rapidly cutting relatively thick materials of varioustypes, with a significant reduction in power requirements in comparisonto known cutting apparatus. It will also be apparent that certainfeatures of my invention can be employed to advantage without acorresponding use of other features.

Referring now more particularly to the drawings and initially to FIGS.l3 thereof, my cutting apparatus includes a rotary shearing apparatusdenoted generally by the reference character 20 and scrap chopping anddisposing apparatus 22. In this arrangement of my invention the cuttingapparatus and scrap disposal mechanism 20--22 is arranged for operationupon a continuously moving length of material supported in this exampleon pass line 24. The pass line typically can be that afforded by aconventional rolling mill (not shown). The maximum plate width isdetermined by the solid outline positions of trimmer housings 26a, 26bwhile the minimum width thereof is denoted by the positions of fixedtrimmer housing 26a and chain outline 28 of the other housing 26b.

The material being cut is supported by a number of rollers 30 located onstandard 32 disposed intermediately of the trimmer housings 26a, 26b.The positions of the movable trimmer housing 26b and the movableintermediate plate standard 32 are adjusted by leadscrew 34 having afirst pitch section 36 and a second pitch section 38. In this example,the pitch sections have a pitch ratio of 1:2 so that the standards 32are moved half the adjusting distance of the movable trimmer housing26b. In consequence, as the lead screw 34 is turned by drive motor 40the plate standard 32 remains midway between the trimmer housings 26a,26b.

Save for the novel and useful adjustment feature described above thecutting apparatus 20-22 is substantially identical on each side ofcenterline 42. Both lateral edges of the material (supported at passline 24) therefore are trimmed simultaneously and in the same manner asdescribed below. Obviously, my novel cutting apparatus can be employedfor trimming a single lateral edge of the material at pass line 24. Thescrap disposal apparatus 22 is likewise provided with fixed and movablehousings 44, 46 between which is disposed a movable support 48 for theaforementioned material. The support 48 and the movable housing 46 canbe adjusted by means of lead screw 50 which can be constructed in amanner similar to the dual pitch lead screw 34.

As better shown in FIG. 1 of the'drawings, the rotary shearing mechanism20 includes a pair of cutting heads 52, 54 disposed adjacent eachlateral edge of the plate material. Each cutting head 52, 54 includes apair of substantially vertically juxtaposed knives 56, 58, or 60, 62,each having a substan- 5 juxtaposable knives can be substituted withinthe principles of my invention, as dictated by specific applicationsthereof.

Each cutting head 52 or 54 (on a selected side of the material to becut) is provided with a drive motor 64 or 66, each of which is coupledto reducer 68 and pinion stand 70 through output shaft 72 or 74. Theoutput shafts 72, 74 are individually connected with similar shafts 72,74 forming part of the cutting apparatus on the other side of thematerial by means of connecting shafts 76, 78 respectively.Complementing the aforementioned tandem connection (transversely of thepass line 24), each pair of adjacent shafts 72, 74 are laterallyinterconnected (parallel to the pass line 24) reducer 68 and pinionstand 70, of otherwise conventional construction. With this arrangementthe circumferential speeds of the eight rotary knives 5662 are preciselysynchronized relative to one another and to the material urged alongpass line 24.

The leading cutting heads 52 are arranged in this example for scoringthe plate or similar material at least to a sufficient depth to cutthrough any work-hardened skin layer or layers of the plate. Infurtherance of this purpose, the leading rotary knives 56, 58 aresomewhat smaller in diameter than the rear ward or parting rotary knives60, 62 as better shown in FIG. 3 of the drawings. In addition, the lowerscoring knife 58 is forwardly offset (.FIG. 3) slightly in advance ofthe upper scoring rotary knife 56. This arrangement facilitatespenetration of the rotary knives 56, 58 into the work hardened skinlayers of the plate or other material.

The shearing knives 60, 62, as noted previously, are of larger diameterthan the scoring knives 56, 58. The lower shearing knife 62 is similarlyand forwardly offset, but to a greater extent, than the leading orscoring rotary knives 56, 58. The greater forward offset is necessitatedby the larger diameter of the rotary knives 60, 62 and their relativelydeeper penetration into the plate being trimmed.

I have determined that my progressive or stepwise cutting operation,involving scoring and subsequent parting or shear ing steps, reduces toa considerable extent the power requirements of the cutting operation.Thicker plate material therefore can be severed with rotary knives thanwas heretofore .possible. The prescoring or definition of the intendedline of cutting or severing by the leading or scoring cutting head orheads 52 leads faster and more accurate parting of the material by thetracking cutting heads 54.

Each of the cutting heads 52, 54 is horizontally adjustable to ensuretracking of the shearing knives 60, 62 relative to the scorings made bythe leading or scoring rotary knives 56, 58. One arrangement foreffecting such adjustment includes a worm gear and screw adjustment,driven by a suitable motor and speed reducer.

In order to accommodate differing thicknesses of plate or othermaterial, means are provided for varying the cutter opening or verticalspacing between each pair of rotary knives 56, 58 or 60, 62. As bettershown in FIGS. 3 and 4 of the drawings, one arrangement of such verticaladjustment means includes at least one, and more advantageously, a pairof eccentrics 80, 82 or 84, 86 mounted on each cutter head 52 or 54. Theaforementioned cutter openings are determined by the rotated positionsof each pair of eccentrics 80, 82 or 84, 86. Each pair of eccentrics areangularly displaced respectively by drive motor 88 or 92 and speedreducer or 94. The output shaft of each reducer 90, or 84 is coupled tothe associated pair of eccentrics 84, 86 or 80, 82 by means of drivepinions, for example, which are enmeshed with driven gears mounted onthe eccentrics.

After complete severing by the shearing knives 60, 62 the trimmings passthrough a roll leveler 96 disposed, in this example, on each side of theplate or to the material and in general alignment with the adjacentrotary knives 56, 58 and 6t), 62. Each roll leveler 96, as better shownin FIG. 3,

better shown in FIGS. 7 and 8 the first two pairs of opposed levelerrollers 100 and 102 are skewed relative to the direction of travel ofthe trimmings to aid in guiding and straightening the scrap. Cooperatingwith the skewed configuration of the opposed rollers 100, 102 of theleveler 96 is the concave contour of the roll faces of the rollers 100,102.

To accommodate differing thicknesses of material, the leveler rollersare mounted for movement toward and away from the scrap pass line 104 asbetter shown in FIGS. 3 and 5 of the drawings. For this purpose thelower rolls of the leveler 96 are mounted on an adjustable table 106.The normal adjustment thereof relative to the pass line 104 iscontrolled by jack screw 108 which in turn is rotated by motor reducerunit 1 10.

To accommodate the normal adjustment of table 106, a curved runout table112 is provided for entry of the scrap. The table 112 extends generallyfrom the cutter opening of the shearing rotary knives 60, 62 to the feedend of the roll leveler 96. To accommodate differing thicknesses ofmaterial and corresponding transverse adjustments of the roller table106, the runout table 112 is pivoted at 114.

At the lower or exit end of the roll leveler 96, a chopping head 116 ispositioned for severing the scrap into convenient lengths to facilitatesubsequent handling. For the latter purpose to chute 118 is juxtaposedto the other side of the chopper head 116.

In my exemplifying arrangement, the chopper head 116 includes a pair ofrotary choppers 120, 122, each of which has a number of chopper blades124, mounted askew about its cirinvention is readily adjustable,moreover, for handling plate or the like materials of differingthicknesses.

Although the invention has been described primarily in connection withtrimming the lateral edges of plate material, it will be understood ofcourse that the invention is of general utility in applications whererapid and accurate cutting is desired. For example, my cuttingapparatus, when arranged as a double head side trimmer for use in a highspeed hot or cold strip mill can be used to advantage for rapidly andaccurately trimming the lateral edges of sheet or strip.

From the foregoing it will be apparent that novel and efficient forms ofdual shearing mechanisms and methods have been disclosed herein. While Ihave shown and described certain presently preferred embodiments of theinvention and have illustrated presently preferred methods of practicingthe same, it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied and practicedwithin the'scope of the following claims.

Iclaim:

1. Cutting apparatus comprising first cutting means for scoring materialto be cut, second cutting means for shearing said material, each of saidfirst and said second cutting means including a pair of laterally offsetand substantially square knife edges, means for mounting said secondcutting means in sub-' stantially tandem alignment with said firstcutting means and in alignment with the scoring of said materialproduced by said first cutting means, and means "for passing saidmaterial successively between the knife edges respectively of said firstand said second cutting means.

2. The combination according to claim 1 wherein each of said first andsaid second cutting means includes a pair of juxcumferential surface.The choppers 120, 122 are generally cylindrical in contour toaccommodate the skewed chopper blades 124.

The scrap cutters 120, 122 are rotated by drive motor 126 coupled tospeed reducer 128. In order to accommodate differing thicknesses ofplate trimmings and the like exiting from the roll leveler 96, thecutter opening between the cutters 120, 122 can be varied by moving oneof the cutters, for example the lower cutter 122 toward and away fromthe upper cutter 120. One arrangement for thus varying the chopperopening includes the apparatus better shown in FIG. 6 of the drawings.The output shaft of speed reducer 128 is coupled to pinion shaft 130mounted generally below the lower cutter 122. Pinion 132 on shaft 130engages ring gear 134 formed in this example integrally on eccentric136. The eccentric 136 serves in addition as a bearing housing for lowercutter shaft 138 which is mounted therein by means of suitable bearings140, 142. The ring gear 134 of the lower eccentric 136 is enmeshed witha similar ring gear 144 of an upper eccentric and bearing housing 146which the upper cutter shaft 148 is similarly mounted on. As shown inFIG. 6 the eccentrics 136, 146 are disposed for a minimum opening 150between the cutters 120, 122. When a half revolution is imparted to theeccentrics 136, 146 by rotation of the pinion shaft 130 the cutteropening 150 assumes its maximum size (not shown).

As better shown in FIG. 6 shaft extensions 152, 154 of the cutter shaft138, 148 respectively are coupled to a suitable gearing train in turnconnected to the output shaft of the speed reducer unit 128 shown inFIG. 3 of the drawings.

My novel cutting apparatus disclosed herein provides rapid and accuratemeans for trimming or otherwise severing plate material. The cuttingapparatus optionally includes novel means for chopping and disposing ofthe scrap that may be produced in the cutting or trimming operation. Inthe latter connection I have disclosed convenient and novel means forstraightening, directing and chopping any such scrap into convenientlengths. Most importantly, my cutting apparatus can accommodate thickerplate material or the like than has been feasible with known equipment.The cutting apparatus of my taposed knives, and means are provided forvarying the opening between each pair of knives for accommodatingdiffering thicknesses of said material.

3. The combination according to claim 1 wherein said material is steelplate having a work hardened skin layer, and said first cutting meansare disposed for scoring said plate so as to penetrate said skin layer.

4. The combination according to claim 1 wherein means are provided foradjustably and laterally moving one of said first and said secondcutting means relative to the other to ensure tracking of said secondcutting means with the scoring produced by said first cutting means.

5. The combination according to claim 1 wherein each of said first andsaid second cutting means include a pair of jux taposed rotatablymounted cylindrical knives.

6. The combination according to claim 2 wherein said opening varyingmeans include at least one eccentric member for each of said first andsaid second cutting means, a knife of each cutting means is mounted uponthe associated eccentric member, and means are provided for angularlydisplacing said eccentric members to vary said openings respectively.

7. The combination according to claim 1 wherein said cutting apparatusis arranged for simultaneously trimming the lateral edges of strip orplate material and includes means for mounting said first and saidsecond cutting means adjacent one of said lateral edges, and additionalfirst and second cutting means are similarly mounted adjacent the otherof said lateral edges.

8. The combination according to claim 7 wherein driving means areprovided for each of said first and said second cutting means, and meansare provided for coupling the outputs of said first cutting meansdriving means respectively together to ensure synchronous engagement ofeach of said first and each of second cutting means with said strip orplate.

9. The combination according to claim 1 wherein each of said first andsaid second cutting means include at least one rotary shear blade, andthe diameter of said first cutting means blade is smaller than that ofsaid secondcutting means blade.

10. The combination according to claim I wherein each of said first andsaid second cutting means includes a pair of juxtaposed rotary shearblades, and one of each of said pairs of blades is forwardly offsetrelative to the other of each pair of opposing directions respectivelyover adjacent surface portions of said material and laterally of saidout line, and directing said pairs of forces successively along said cutline so that said first pair of forces partially cut and said secondforces completely sever said material along said out line.

13. The method according to claim 12 wherein said material has ahardened skin layer and including the additional step of setting themagnitude of said first force pair at least to penetrate the hardenedlayer of said material.

1. Cutting apparatus comprising first cutting means for scoring materialto be cut, second cutting means for shearing said material, each of saidfirst and said second cutting means including a pair of laterally offsetand substantially square knife edges, means for mounting said secondcutting means in substantially tandem alignment with said first cuttingmeans and in alignment with the scoring of said material produced bysaid first cutting means, and means for passing said materialsuccessively between the knife edges respectivEly of said first and saidsecond cutting means.
 2. The combination according to claim 1 whereineach of said first and said second cutting means includes a pair ofjuxtaposed knives, and means are provided for varying the openingbetween each pair of knives for accommodating differing thicknesses ofsaid material.
 3. The combination according to claim 1 wherein saidmaterial is steel plate having a work hardened skin layer, and saidfirst cutting means are disposed for scoring said plate so as topenetrate said skin layer.
 4. The combination according to claim 1wherein means are provided for adjustably and laterally moving one ofsaid first and said second cutting means relative to the other to ensuretracking of said second cutting means with the scoring produced by saidfirst cutting means.
 5. The combination according to claim 1 whereineach of said first and said second cutting means include a pair ofjuxtaposed rotatably mounted cylindrical knives.
 6. The combinationaccording to claim 2 wherein said opening varying means include at leastone eccentric member for each of said first and said second cuttingmeans, a knife of each cutting means is mounted upon the associatedeccentric member, and means are provided for angularly displacing saideccentric members to vary said openings respectively.
 7. The combinationaccording to claim 1 wherein said cutting apparatus is arranged forsimultaneously trimming the lateral edges of strip or plate material andincludes means for mounting said first and said second cutting meansadjacent one of said lateral edges, and additional first and secondcutting means are similarly mounted adjacent the other of said lateraledges.
 8. The combination according to claim 7 wherein driving means areprovided for each of said first and said second cutting means, and meansare provided for coupling the outputs of said first cutting meansdriving means respectively together to ensure synchronous engagement ofeach of said first and each of second cutting means with said strip orplate.
 9. The combination according to claim 1 wherein each of saidfirst and said second cutting means include at least one rotary shearblade, and the diameter of said first cutting means blade is smallerthan that of said second cutting means blade.
 10. The combinationaccording to claim 1 wherein each of said first and said second cuttingmeans includes a pair of juxtaposed rotary shear blades, and one of eachof said pairs of blades is forwardly offset relative to the other ofeach pair of blades and relative to the travel direction of saidmaterial therebetween.
 11. The combination according to claim 10 whereinthe forward offset of said second cutting means blades is greater thanthe forward offset of said first cutting means blades.
 12. A method forcutting material, said method comprising the steps of applying first andsecond pairs of opposing forces to said material, magnitude theapplication of the opposing forces of each force pair relative to anintended cut line in said material, spreading the opposing forces ofeach force pair in opposing directions respectively over adjacentsurface portions of said material and laterally of said cut line, anddirecting said pairs of forces successively along said cut line so thatsaid first pair of forces partially cut and said second forcescompletely sever said material along said cut line.
 13. The methodaccording to claim 12 wherein said material has a hardened skin layerand including the additional step of setting the magnitude of said firstforce pair at least to penetrate the hardened layer of said material.